Apparatus for high frequency dielectric heating of condenser bushings



Jan. 4, 1949. c. J. MADSEN APPARATUS FOR HIGH FREQUENCY DIELECTRICHEATING 0F CONDENSER BUSHINGS Flled April 3, 1946 .Y wMzMo m n ,j dv 0 C& a s w W Patented Jan. 4, 1949 APPARATUS FOR HIGH FREQUENCY DI-ELECTRIC HEATING OF CONDENSER BUSHINGS Carl J. Madsen, Wilkinsburg',Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh,Pa., a corporation of Pennsylvania.

Application April 3, 1946, Serial No. 659,227

12 Claims. 1

Broadly, my invention relates to the simultaneous or selective heatingof one or more loads, which may be alike or dilierent, by means ofelectrical power derived from a high-frequency source such as atube-oscillator generator. More particularly, my invention relates tothe dielectric heating of one or more condenser bushings such as is usedto pass high'tension electrical current through containers or walls forelectrical apparatus.

An object of my invention is to provide a system in which a singletube-oscillator generator and a single high-frequency transmission linecan be used to heat a plurality of dielectric loads of the same ordifferent impedances at the same or difierent times.

Another object of my invention is to improve upon the treatment ofcondenser bushings which are dried, gas-exhausted and oil-filled duringtheir manufacture.

An ancillary object of my invention is toimprove upon the condenserbushing treating means and method disclosed in the copending patentapplication of H. J. Lingal and T. R. Watts, Serial No. 488,634,filedMay 27, 1943.

My invention is of general application, and other objects, features andinnovations of my invention, in addition to the foregoing, will bediscernible from the following description of a preferred embodiment.The description is to be taken in conjunction with the accompanyingschematic drawing which illustrates, in a general way understandable inthe art, a system for heating a plurality of condenser bushings.

For convenience, the bushings are assumed to be of the same generalcharacter, and in the drawing each condenser bushing 2 comprises anouter porcelain casing 4 through which passes an axial bushing-stud orconductor 6 having threaded ends 8 and I0. Wrapped around theconductor 6is a central capacitance wrapping l2 which tapers to the ends 8 and Ill,and is spaced from the casing 4. The casing 4 also tapers toward theends of the condenser bushing, and comprises upper and lower porcelainshells I 4 and [6, respectively, separated by a central comparativelynarrow metallic mounting-fitting or flange structure I8 by which thecondenser bushing is supported in a customary manner when in use. Theother ends of the shells II and I6 are maintained in position by anupper metal cap 20 and a lower metal fitting 22, both of which areelectrically secured to the conductor 6. The insulating wrapping l2 maycomprise alternate layers of metal foil and dielectric material, such aspaper, ar-

. 2 ranged to form a capacitor which grades the voltage stresses betweenthe central conductor 6 and the ground potential of the mounting fittingl8 which is usually secured to a Wall of a grounded metal tank orsimilar part.

In making high-voltage condenser bushings, it is customary to dry thecapacitance wrappings under vacuum, and in some designs to fill themwith oil or other suitable substance for preventing the absorption ofWater during the useful life of the bushings. The oil impregnates thecapacitance wrapping, and in order to secure an efiicient low-losscondenser bushing, the impregnation should be carried out in a mannerwhich removes entrapped moisture and air as completely as possible, Theaforesaid patent application describes a satisfactory process usingsteam for treating condenser bushings. The use of steam necessitates asteam piping system and a hollow axial conductor for the condenserbushing.

In accordance with my invention, the heating is performed withhigh-frequency currents. This means that no steam source or pipedelivery is required, and the heating is accomplished in a matter ofminutes, rather than hours. Even more important, it means that the axialconductor of the condenser bushing can be either solid or hollow.

In the diagrammatic form of my invention shown in the drawing, thesource of power for the heating is generated by any suitabletubeoscillator generator 28 which is provided with its ownfrequency-determining circuit. Power from the generator is distributedby a coaxial cable transmission line 30 which comprises an outergrounded hollow metal tubular conductor 32 containing an insulatedcentral conductor 34. At spaced points along the transmission line,heating stations 36 are provided at each of which a condenser can betreated.

Eachstation includes a tuning unit 38 com prising a grounded shieldingbox 40 having suitable holes or other expedients through whichcontrolling elements, mounted on the face of the box, and insulatedconductors pass to parts inside. The tuning unit comprises a variablecoupling transformer 42 and a variable tuning reactor 44, in the form ofa variable induction coil, supported inside the box. The couplingtransformer comprises a primary winding 46 and a closely coupledsecondary winding 48. Electrically the primary winding is'in a primarycircuit connected between ground, or the ground conductor 32, and. theinsulated conductor 34 of the coaxial cable. Accordingly, the tuningunits are separately branched from the coaxial cable 30. The secondarywinding 48 is in a secondary circuit which includes, in series insidethe box 40, the tuning reactor 44 and an ammeter 50, The ammeter end ofthe secondary circuit is grounded and the other end is connected to acontact 54 of the equivalent of a single-pole double-throw switch havinga blade 55. The blade 56 is connected to.

an insulated outlet conductor 58 which, in turn, is connected to thecentral mounting flange I8 of the condenser bushing. The other contact60 of the switch is connected to a power factor measuring device 62 forascertaining the power factor of the condenser bushing. The electricalconnections for heating the condenser bushing are com pleted byconnecting both ends 8 and ID of the conductor 6 to ground by means offlexible detachable straps 64 and 66, respectively.

When the switch blade 55 engages the contact 54, power is delivered tothe condenser bush ing for dielectrically heating its capacitancewrapping I2 which acts as a dielectric load. In order to obtain anappreciable amount of powerdelivery, the tuning unit 38 is manipulatedto bring the secondary circuit with the condenser bushing in series intoresonance with the frequency of the power supply. The transformer 42 issuch as to introduce a high primary impedance when the secondary circuitis detuned from resonance, or is otherwise deenergized as, for example,by disengaging the blade 56 from its contact 54. By this means, it ispossible to apply or remove power to any one condenser bushing withouteffecting the heat-treatment of the others which also derive power fromthe transmission line 30. It is also possible to measure the powerfactor of the condenser bushing for ascertaining its condition at anytime, by moving the blade 56 to the contact 60.

For degassing the condenser bushings during the high-frequency heatingand subsequent application of oil to the insides of the bushings, pipeconnections are provided. For exhausting gas, a common vacuum line 68 isprovided having a plurality of branch pipes each of which includes ashut-off valve 12. A connection pipe 14, part of which may be flexibleif desired, can be secured to the cap 20 of each condenser bushing, andextends to the associated branch pipe 10. Similarly, for selectivelyfilling the condenser bushings with oil, a common oil supply pipe 16 isprovided which can be connected to the bottom of the several condenserbushings through branch pipes 18, each having a shut-off valve 80. Acommon oil-return line 82 is provided having a plurality of branch pipes84 connected to the connection pipes 14, each branch pipe 84 including ashut-off valve 85.

The condenser bushings may be treated individually or collectively inany grouping, along the lines described in the aforesaid Lingal andWatts patent-application; a valve or valves 12 being open and theassociated oil shut-off valves 80 and 86 closed during the initialgas-evacuation, and the valves 80 and 86 open during oiltreatment.

By providing individual tuning units for the several condenser bushings,it is possible to treat the condenser bushings individually,irrespective of variations in capacitance so long as the associatedtuning unit, with the condenser bushing connected, can be tunedsubstantially to resonance. The magnetic coupling between the windingsof the'transformer should preferably be variable so that each of thebushings in various stages of completion (including some filled withoil) may be heated at its proper rate, irrespective of the of completionwhich has been reached by any other bushing. The high primary impedanceof each transformer permits any condenser bushing to be connected to anddisconnected from the power source at any time, no matter what thecondition is of any other condenser bushing.

be moved.

The drawing also slzetchedly illustrates equipment for convenientlytreating the condenser bushings. A metallic cage structure is providedhaving a plurality of partitions 83 dividing it into plurality ofcubicles in each of which a condenser bushing is vertically supported.Consequently each cubicle can be considered a heating station 36. Heavycopper bus bars 90 and 92, to which the straps 64 and 68 may be secured,run horizontally through the cubicles, being located so as to be nearthe caps 20 and fittings 22 of the condenser bushings A verticalequalizing jumper bar 94 is provided in each cubicle, extending betweenthe upper and lower bus bars. These bars are in intimate contact withthe metallic cage structure. The coaxial cable 30 also passeshorizontally along a side of the cage structure, and between thehorizontal bus bars 90 and 92 so as to be conveniently located withrespect to the tuning units 33 which are supported on the walls of eachcubicle in the manner so that the outlet conductors 58 can convenientlybe connected to themounting flanges N3 of the condenser bushings. Thecommon pipes for gas-evacuating and oil-filling the condenser bushingsrun along the top and bottom of the cage structure so as not tointerfere with movement of the condenser bushings.

The back of each cubicle is provided with a door 96 through which acondenser bushing can Preferably the coaxial cable kept filled with aninert gas such as nitrogen, and copper is the preferred material for allcurrentcarrying parts of the treating apparatus. The

cage structure is preferably thoroughly grounded; and various bus bars,the tuning units and the outer conductor of the coaxial cable arepreferably secured to the cage structure so as to make goodelectricalcontact with it. In order to prevent localized heating in eachcondenser bushing, copper should also be used in the connection from theouter foil layer or layers of the wrapping [2 to the fitting l8.

The frequency for heating largasize oil-filled condenser bushingsusually runs into megacycles. Exceptionally high frequencies, althoughusable, are not recommended because of the possibility of obnoxiousstanding waves. Five megacycles have been found satisfactory for heating138 kv. condenser bushings of both the dry paper and oilimpregnatedpaper types, such bushings standing about 10 feet in height. However,other frequencies more and less than five megacycles can be used.

While I have described my invention in connection with a preferred form,it is obvious that its teachings are of general application tohighfrequency heating, as well as being of special merit for treatingcondenser bushings.

I claim as my invention:

1. A dielectric heating system for treating a plurality oi condenserbushings of different sizes and of a type having a central conductorhaving opposite ends/a capacitive wrapping about said conductorIandanouter mounting fitting about said wrapping and lying between saidconductorends, said system comprising a high-frequency generator, aplurality of heating stations, each adapted to receive a condenserbushing, a commen power-transmission line extending from said generatorto each of said stations, each station comprising a coupling transformerhaving a high impedance primary winding and a secondary winding, circuitmeans at each station connecting each primary winding to saidtransmission line, with the primary windings of the several stations inparallel, connection means at each station for connecting the associatedsecondary winding thereat across said central conductor and saidmounting fitting of the condenser bushing at the associated station, thelast said connection means comprising a variable inductor, the last saidconnection means at each station comprising a pair of conductors at acommon potential, adapted to be connected to the opposite ends of saidcentral conductor, and comprising an insulated conductor adapted to beconnected to said mounting fitting.

2. A dielectric heating system for treating a plurality of condenserbushings of difierent sizes and of a type having a central conductorhaving opposite ends, a capacitive wrapping about said conductor and anouter mounting fitting about said wrapping and lying between saidconductorends, said system comprising a high-frequency generator, aplurality of heating stations, each adapted to receive a condenserbushing, a common power-transmission line extending from said generatorto each of said stations, each station comprising a coupling transformerhaving a high impedance primary winding and a secondary winding, circuitmeans at each station connecting' each primary winding to saidtransmission line, with the primary windings of the several stations inparallel, connection means at each station for connecting the associatedsecondary winding thereat across said central conduotor and saidmounting fitting of the condenser bushing at the associated station, thelast said connection means comprising a variable inductor, a cagestructure at each of said heating stations, each cage structure havingmeans for receiving a condenser bushing for dielectric heating, the lastsaid connection means comprising a conductor in intimate electricalcontact with said cage structure.

3. An invention including that of claim 2 but further characterized bysaid transmission line comprising a coaxial cable having an outerconductor, a metallic box at each station in intimate electrical contactwith the associated cage structure, each box carrying therein theassociated transformer and variable inductor, said outer conductor andan end of the primary winding of each transformer being substantiallydirectly electrically connected to the box associated therewith at aheating station.

4. A system for treating a condenser bushing comprising a centralbushing-conductor, a tapered capacitive wrapping about saidbushingconductor, and a mounting fitting of lesser length than saidcentral bushing-conductor, said mounting fitting being about saidcapacitive wrapping; said system comprising a tube-oscillator generator;a metallic cage-structure spaced from said generator; a high-frequencytransmission line from said generator to said cage-structure; ashielding box carried by said cage-structure and having therein atransformer means comprising a high impedance primary winding and asecondary winding; a primary circuit connected to said transmissionline, comprising an insulated conductor insulatedly passing into saidbox and connected to said primary windin a secondary circuit comprising,in series, said secondary winding, a tuning means, a switch means, andan insulated energizing conductor; said tuning means, switch means andenergizing conduc tor comprising portions in said shielding box;conductor means for connecting a grounded end of said secondary windingto said central bushing-conductor; and an intermediate conductor forelectrically connecting said energizing conductor to said mountingfitting of the condenser bushing.

5. A system for treating a condenser bushing comprising a centralbushing-conductor, a ta, pered capacitive wrapping about said bushingconductor, and a mounting fitting of lesser length than said centralbushing-conductor, said mounting fitting being about said capacitivewrapping; said system comprising a tube-oscillator generator; a metalliccage-structure spaced from said generator; a high-frequency transmissionline from said generator to said cage-structure; a shielding box carriedby said cage-structure and having therein a transformer means comprisinga high impedance primary winding and a second ary winding; a primarycircuit connected to said transmission line comprising an insulatedconductor insulatedly passing into said box and connected to saidprimary winding; a secondary circuit comprising, in series, saidsecondary winding, a tuning means, a switch means, and an insulatedenergizing conductor; said tuning means, switch means and energizingconductor comprising portions in said shielding box; means comprising aconductor on said cage-structure for connecting an end of said secondarywinding to said central bushing-conductor; an intermediate conductor forelectrically connecting said energizing conductor to said mountingfitting of the condenser bushing; a circuit in said shielding boxincluding power factor measuring means having an indicator observableexternally of said cagestructure; said switch means having an operatingmember outside said cage-structure for disconnecting said energizing andintermediate conductors from said secondary circuit and connecting themin said power factor measuring circuit,

6. A treating system for a plurality of condenser bushings, eachcomprising a central bushingconductor having ends, a capacitive wrappingaround said central bushing-conductor, and a mounting fitting about saidcapacitive wrapping, said mounting fitting lying between saidconductor-ends; said system comprising a tube-oscilla-- tor generator; acage-like structure divided into a plurality of cubicles, each of whichcan receive a condenser bushing; a high-frequency transmission lineextending along said cage-structure; each cubicle carrying a transformermeans comprising a high impedance primary winding and a secondarywinding; connecting means for each cubicle for connecting the associatedprimary winding to said transmission line; a secondary circuit for eachcubicle comprising the associated secondary winding, branch conductorsspaced in each cubicle for electrical connection to the ends of saidcentral bushing-conductor, and an intermediate conductor between saidbranch conductors and insulated therefrom for electrical con-- nectionto the mounting fitting of said condenser bushing adapted to beassociated therewith; said secondary circuit including a circuit tuningmeans in series with the intermediate conductor and the secondarywinding in the same cubicle.

7. An invention including that of claim 6 characterized by said spacedbranch conductors comprising substantially parallel bus-bars, eachbusbar being common to said cubicles and extending across said cage-likestructure in intimate electrical contact therewith.

8. An invention in accordance with claim 6 but further characterized byeach cubicle having a switch means in the associated secondary circuitand a power factor measuring circuit; said switch means of each cubiclebeing constructed and arranged for disconnecting the associated branchconductors and intermediate conductor from the associated secondarycircuit and connecting them in said power factor measuring circuit.

9. A heating station for drying condenser bushings of a type comprisingan axial central conductor, a capacitance wrapping about said centralconductor, and a mounting fitting centrally circumferentially aroundsaid wrapping, said station comprising a cage-like cubicle of metallicWalls adapted to vertically receive a condenser bushing, a pair ofspaced :bus-bars carried by said cubicle in intimate contact therewith,said bus-bars having a conductor-portion attached thereto of limitedlength for connection to an end of the central conductor of saidcondenser bushing, and an intermediate conductor of limited lengthbetween said bus-bars for connection to the mounting fitting of thecondenser bushing, said intermediate conductor being relatively insulated with respect to said cubicle, said cubicle having a spacetherein for vertically receiving said condenser bushing with itsmounting fitting insulated from the metallic walls of the cubicle.

10. An invention including that of claim 9 but further characterized bysaid bus-bars being vertically spaced and substantially horizontal, onebus-bar being near the bottom of the cubicle and the other bus-bar nearthe top of the cubicle.

11. An invention including that of claim 4 but further characterized bysaid conductor means comprising vertically spaced substantiallyhorizontal bus-bars, each bus-bar being in intimate electrical contactwith the cage-structure.

12. A heating station for drying condenser bushings of the typecomprising an axial central conductor, a capacitance wrapping about saidcentral conductor, and a fitting mounted centrally circumferentiallyaround said wrapping, said station comprising a cage-like cubicle ofmetallic walls adapted to receive a condenser bushing therein, a pair ofspaced bus-bars secured to said walls in intimate electrical contact 8therewith, each of said bus-bars having a conductor-portion attachedthereto of limited length for connection to the ends of the centralconductor of said condenser bushing, an insulated intermediate conductorof limited length inside said cubicle for connection to the mountingfitting of said condenser bushing, transformer means fo providing a highfrequency voltage across said intermediate conductor and saidconductor-portions, said cubicle having a space therein for receivingthe condenser bushing With its mounting fitting insulated from themetallic walls of the cubicle and with the ends of its central conductorposition to be reached by said limited conductor-portions and itsfitting positioned to be reached by said insulated inter mediateconductor.

CARL J. MADSEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,374,679 Pratt Apr. 12, 19211,582,720 Zworykin Apr. 27, 1926 1,871,269 Hobrock Aug. 9, 19321,891,699 Viry Dec. 20, 1932 1,959,390 Smith May 22, 1934 2,112,418 Hartet al Mar. 29, 1938 2,147,689 Chaffee Feb. 21, 1939 2,179,261 KellerNov. 7, 1939 2,248,840 Wilkoff 1- July 8, 1941 2,261,847 Dufour et a1Nov. 4, 1941 2,263,681 Hart Nov. 25, 1941 2,293,851 Rogers Aug. 25, 19422,308,043 Bierwirth Jan. 12, 1943 2,321,130 Crandell June 8, 19432,321,189 Dravneek June 8, 1943 2,322,903 Wilkoir June 29, 19432,381,323 Vore Aug. 7, 1945 2,388,824 Brown Nov. 13, 1945 2,395,920Grotenhuis Mar. 5, 1946 2,401,277 Stratton May 28, 1946 FOREIGN PATENTSNumber Country Date 457,381 Great Britain Nov. 26, 1936 OTHER REFERENCESModern Plastics, June 1944, pages 116418, 166, 168,

